Meteorology 302.01 Spring 2004

Class Notes for 2/11

Herman Paule Jr. & Lea Reyes

Assignments

Sheets and Williams: Preface; Chapter 1
Williams, Up through Page 36; 63-71; Chapter 9, 139-147; Chapter 12

Housekeeping

- Distributed and looked at US surface weather map for 02/11/04 at 12Z (4:00AM PST).

- All former web links the class has looked at during previous meetings that appear to be missing have been transferred to the “Completed Links” link.

Definitions

Pressure – Weight of the atmosphere above the level it is measured; composed of a column of air from sea level to the atmosphere.

“Atmospheric Bricks” – an arbitrarily sized column of air encompassing many molecules of air.

Air Parcels – Packed air (shoeboxes).

Barometer – An instrument used to measure pressure.

Bar – A unit of measurement for pressure; equivalent to a dine per square centimeter.

Iso-Bars – Lines on a weather map, drawn in 4 millibar intervals, connecting points with equal pressure (blue lines).

How To Measure Pressure From A Weather Map

1. Look at a weather station on a weather map.
2. Find the three-digit number on the upper-right corner.
3. If the first two digits are greater than 60, then place a decimal point after the second number and place a “9” before the first number (i.e. If 860 is shown, then the pressure is 986.0 millibars).
4. If the first two digits are less than 60, then place a decimal point after the second number and place a “10” before the first number (i.e. If 320 is shown, then the pressure is 1032.0 millibars).
5. The units of pressure on a weather map should be in millibars.

Pressure Around The World

- Average sea level pressure = 1000.0 millibars

- Average pressure on Mars = 5.20 millibars

- Average pressure at Tahoe = 850 millibars

- Average pressure at Everest = 300 millibars

Facts About Pressure

- Pressure changes all the time (varies with the wind).

- Pressure of Earth’s atmosphere ranges from 960.0 to 1060.0 millibars.

- Rule of Thumb: On a given level, in the absence of other effects, air locally tends to move from regions with higher values of pressure to regions with lower values of pressure.

Pressure On A Weather Map

- Circles on a weather map show were pressure is highest and lowest.

- Low-pressure zones are indicated with a red “L” (“LOW” is written if space permits).

- High-pressure zones are indicated with a blue “H” (“HIGH” is written if space permits).

- The closer the iso-bars are to each other, the stronger the winds in that particular region.

Movie/Window Example

In the particular movie, the alien and the woman are in the spaceship together in space. The woman opens the window in the spaceship, and everything, including the alien, starts flying out. The amount of air (pressure) is greater inside the ship than outside. When the window is opened, the air quickly rushes from an area of higher pressure to lower pressure. The wind is faster if the difference in pressure is greater.